1. Field of the Invention
The present invention relates generally to the fields of surface chemistry and self-assembly materials. More particularly, the present invention relates to the design and synthesis of a stable and reversible system containing metal, alloy, or core/shell nanoparticles coated with a material that prevents irreversible aggregation of the nanoparticles and offers the possibility of radical initiation to effect tailored polymer growth from the surface.
2. Description of the Related Art
Metal and alloy nanoparticles, especially those comprised of noble metals in discrete form or in shell/core architectures, have been the focus of recent interest with regard to their potential applications in areas such as electronics, optics, biotechnology, and chemical catalysis. This heightened interest is mainly due to the size-dependent and shape-dependent optical and electrical properties and the amenability for the modification of the particle surface by taking advantage of strong ligand-metal interactions. The chemisorption of small molecules on the surface of metal nanoparticles (MNPs) frequently causes irreversible aggregation of the nanoparticles, and the resultant aggregates show distinct electronic, optical and biological properties from the individual metal nanoparticles. The change in these properties due to irreversible aggregation can be a major drawback for further practical applications owing to the unpredictable character of the resulting aggregates; however, controllable aggregation can be utilized in various technical applications, such as the assembly of nanoparticles, nanodevices, and colloidal sensors.
Metal nanoparticles with a radius much smaller than the incident wavelength of the light strongly absorb at certain wavelengths due to the resonance excitation of the surface plasmons, and these absorption bands are influenced by particle aggregation. When the metal nanoparticles aggregate, the distance between the particles becomes smaller, and the surface plasmon bands shift to longer wavelengths than those of the individual particles. The red shift of the metal nanoparticles can often be followed with the naked eye; for example, in the case of gold nanoparticles (AuNPs), the solution changes from pink-red to purple-blue. These color changes induced by the shorter interparticle distances provide a simple but effective method as a practical colorimetric tool for detecting specific reactions between anchored molecules on the gold nanoparticles and receptor molecules in the solution. Using these properties, it has been shown that gold nanoparticles modified with oligonucleotides aggregate through the hybridization of complementary oligonucleotide strands, providing a practical tool for the detection of targeted DNA sequences. Others have demonstrated the analytical capabilities of protein A-coated gold nanoparticles to determine the level of anti-protein A in serum samples. More recently, it has been shown that functionalizing gold nanoparticles with 2,2′-bipyridine and further complexing these nanoparticles with lanthanide metal ions such as europium and terbium could activate them as sensors for biologically important cations. Along with these specific examples, various approaches have been reported that demonstrate AuNP aggregates via hydrogen bonding, metal-ligand interactions, and ion paring.
The controlled reversible aggregation/deaggregation of metal nanoparticles is an important feature, especially where repeated use and in situ feedback are desired. Nevertheless, reversibility is difficult to realize as the aggregated metal nanoparticles tend to collapse and fuse irreversibly into larger particles. In most cases, the linkages between the aggregated metal nanoparticles cannot be separated to yield the initial constituent particles. To address this problem at least in part, external stimuli have been used to affect the aggregation/deaggregation of metal nanoparticles peripherally. These approaches can be broadly classified into three categories: temperature changes, pH changes, and molecular recognition.
Despite the significant amount of work on reversible processes that has been performed, more general and nonspecific routes are required to overcome the restrictions associated with the specific chemical routes for the development of aggregation-based sensors and the controlled assembly of metal nanoparticles for optoelectronics applications.
Thus, there is a recognized need in the art to design and synthesize metal nanoparticles functionalized with rationally designed coatings that enable their reversible aggregation/deaggregation. More specifically, the prior art is deficient in utilizing surface chemistry and coatings to affect reversible aggregation/deaggregation of metal nanoparticles. The present invention fulfills this long-standing need and desire in the art.